Monday was free Slurpee day at 7-Eleven locations across the country, but some Slurpee fans found that all the free frozen drinks had already been slurped up.

The company has been serving free 7.11-once Slurpees on July 11 (7-11) since 2002, but apparently demand this year was higher than ever. The company estimated that more than 5 million free Slurpees were served, compared to 3 million in 2009.

"We knew there would be great interest and customer participation, but the number of customers who came to our stores for free Slurpee drinks surpassed our expectations," said Margaret Chabris, director of corporate communications for 7-Eleven Inc.

Customers expressed their disappointment on Twitter and Facebook when they didn’t get their free Slurpees. One of my friends sent me a picture from a Manhattan 7-Eleven in Gramercy Park. In the picture, a sign read, “No more free Slurpees.” He is a passionate Slurpee fan, so I can only imagine his disappointment.

Granted, the Dallas-based company did say the free Slurpees were only going to be given out “as supplies last.” Stores mainly ran out of the special-sized free Slurpee cups, Chabris said

The company did apologize to fans on its Twitter page, giving them the number to its guest relations hotline (1-800-255-0711) so they could receive a free coupon.

Even though it seems like the company is giving away a lot of free items, an article in USA TODAY commented on the power of free. It said that when the company gave away 4.5 million Slurpees on 7-11-2010, actual Slurpee sales increased 38 percent for the day. Apparently, the 7.11-once cups aren’t enough once Slurpee fans get a taste. Customers are also willing to go out of their way to buy things like gas, just for a free cup of flavored corn-syrup and ice.

Chabris said the company is preparing for the demand of free Slurpee day next year.

"We are gathering information and ideas to help us better prepare for 7-Eleven Day next year, when the company turns 85," Chabris said.